1. Field of the Invention
The present invention relates to a beam scan type recording apparatus such as a laser printer.
2. Description of the Related Art
In a conventional laser printer, a so-called f.theta. lens is used for scanning a laser beam deflected by a polygon mirror at a uniform speed on a photosensitive medium. However, the f.theta. lens has such disadvantages that it is expensive.
In order to solve this problem, several apparatuses without the f.theta. lens have been proposed as follows.
For example, in a beam scan apparatus disclosed in the Japanese patent laid open publication (JP-A) No. 61-173573, the time interval between successive emission timings of the beam is controlled in stead of using the f.theta. lens. In order for that, the beam scan apparatus comprises a ROM for storing time interval data between emission timings of the beam corresponding to respective exposure points. The time interval data stored in the ROM are converted into an analog voltage signal, and the signal is converted into a frequency signal for driving a laser diode by a voltage to frequency (V/f) converter.
Also, in an apparatus disclosed in the Japanese patent laid open publication (JP-A) No. 55-25081, the above time interval data stored in the ROM are also converted into an analog voltage signal by a digital to analog converter, and frequency-modulated pulses for driving a laser diode are generated by a voltage control type oscillator according to the converted analog voltage signal so as to vary respective time intervals therebetween.
In these apparatuses, since the time interval between successive emission timings of the beam is converted by an analog electric circuit, the time interval therebetween may be changed due to a thermal drift caused in the circuit, resulting in that these apparatuses are not suitable for a practical use.